1. Field of the Invention
The present invention relates to a modulation apparatus for modulating an extreme high frequency radio wave such as a millimeter wave.
2. Related Background Art
Conventionally, in order to modulate an extreme high frequency radio wave such as millimeter wave, a modulation is performed by controlling impedance being a partial structural element of a waveguide path (including a waveguide, a plane waveguide path or the like) which is a path of the radio wave in a transmitter in response to a signal to be transmitted and varying an amplitude of the radio wave to be outputted.
As an example of modulation technique of this system, a system using a PIN diode as a variable impedance element has been known. For example, such the system is described in detail in "NRD Guide Digital Transceiver for Millimeter Wave LAN System", IEICE TRANS. COMMUN., Vol. E79-B No. 12 December 1996.
This thesis describes a communication system using an NRD guide (nonradiative dielectric waveguide) as a communication medium.
FIG. 5 is a perspective view showing basic structure of the NRD guide.
The NRD guide, which has an upper conductive plate 101 and a lower conductive plate 102 mutually located in parallel, is mediately composed of a dielectric strip 103 constituting a dielectric line between the conductive plates 101 and 102. The conductive plates 101 and 102 are formed out of plane metal plates and the dielectric strip 103 is formed out of a fluororesin.
If an interval between the conductive plates 101 and 102 is maintained to be less than or equal to half a free-space wavelength of a frequency to be used, the transference of an electromagnetic wave in polarization parallel to the wall is interrupted and thus the electromagnetic wave is transferred along the dielectric strip 103. Preferably, the width of the dielectric strip 103 is set to be substantially equal to half the above-described wavelength.
FIG. 6A shows the structure of a PIN diode modulator described in the above thesis.
Metal plates 61 and 62 correspond to the conductive plates 101 and 102 respectively. A dielectric strip 63 corresponds to the dielectric strip 103 shown in FIG. 5. An electromagnetic wave RF 64 is transferred along the dielectric strip 63. Reference numeral 65 denotes a modulated electromagnetic wave, 66 denotes an air gap, 67 denotes a dielectric piece, 68 denotes a high-permittivity sheet and 69 denotes a beamlead diode mount.
FIG. 6B shows detailed structure of the beamlead diode mount 69.
Reference numeral 71 denotes a beamlead-type GaAs PIN diode, 72 denotes a pair of electrodes, and 73 denotes a bias choke.
The PIN diode 71 featuring high frequency operation is bounded between the electrodes 72 in the NRD guide beamlead diode mount 69. The mount 69 is sandwiched in the dielectric strip 63. Reflected power from the PIN diode 71 with high level of a digital signal can be reduced by placing the high-permittivity sheet 68 in front of the mount 69. The air gap 66 placed in the dielectric strip 63 acts as a reactive element. This gap becomes a useful tool to completely suppress residual reflection. Using such matching technique, the transmitting waves can be passed or reflected depending on the applied high and low levels of the digital signals to the PIN diode 71. The reflected power is absorbed by matched load connected with a circulator.
Subsequently, the principle of a modulator called as a reflection-type modulator in the related background art will be explained.
In FIG. 4, a waveguide path 21 acts as an input unit for inputting the millimeter wave before being modulated and an output unit for outputting the millimeter wave after being modulated. A pair of electrodes 22 arranged at an edge of a matching circuit 27 also acts as a pad to hold a diode. Reference numeral 23 denotes a PIN diode. A switching circuit is formed by the electrodes 22 and the PIN diode 23. Reference numeral 24 denotes a source of a transmission information signal modulated to be transmitted. Reference numeral 25 denotes a high-frequency choke. The matching circuit 27 performs matching with the switching circuit arranged at an edge of the waveguide path 21. Reference numeral 28 denotes a reflectionless termination.
In this structure, the millimeter wave inputted to the waveguide 21 is applied to the switching unit, which is composed of the electrodes 22 and the PIN diode 23, through the matching circuit 27.
On the other hand, signal voltage to be transferred is applied to the PIN diode 23 from the signal source 24 through the high-frequency choke 25 to act as a vias voltage for the diode. Therefore, at the time when a positive bias is supplied to the diode 23, the diode is in a conductive condition. At the time when a zero or inverse bias is supplied to the diode 23, the diode is in an interruptive condition. That is, a switching operation is conducted according to the signal.
When the diode 23 is in the conductive condition, if the matching circuit 27 is arranged such that characteristic impedance of the waveguide 21 is matched with characteristic impedance of a portion including the PIN diode 23, the electrodes 22 and the reflectionless termination 28, then an input millimeter wave is absorbed in the reflectionless termination 28, and any reflection does not occur. When the diode 23 is in the interruptive condition, mismatch occurs and the input millimeter wave is reflected and returned to the waveguide 21.
The reflection-type modulator is constituted as above.
Since the PIN diode does not actually perform a detecting operation in a millimeter wave band, the reflectionless termination is indispensable in the conductive condition.
However, in this reflection-type modulator, such structure as preventing secondary reflection to an input side is indispensable by arranging the reflectionless termination 28 behind the diode 23, thereby resulting in complicated structure.
Also, the PIN diode operating in the millimeter wave band is ordinarily made by a diode such as GaAs or the like, thereby resulting in relatively high cost.